1. Field of the Invention
The present invention relates to an inkjet printing apparatus which forms an image by reciprocally scanning a print medium by a printhead a plurality of times, and a printing method therefor.
2. Description of the Related Art
Recently, with the increasing popularity of information processing apparatuses such as personal computers, various types of printing apparatuses capable of printing images have become available, and their performance has rapidly improved. Part of the mainstream of these printing apparatuses is an inkjet printing apparatus which prints on a print medium such as paper, cloth, or an OHP sheet by discharging ink from orifices. The inkjet printing apparatus has great advantages: it can perform non-impact low-noise printing, it can print at high density and high speed, it can easily achieve color printing, and its cost is low.
The inkjet printing apparatus having these advantages has improved the image quality by increasing the resolution of a printed image using smaller ink droplets. The inkjet printing apparatus has prevailed even in a field where high-quality large-size outputs such as a large-format photograph, poster, CG, and CAD are required. Speeding up of the inkjet printing apparatus is demanded while suppressing a decrease in throughput caused by upsizing.
To output a color image at high speed, so-called 1-pass printing is preferably performed to form an image in an area covered by one scanning of the printhead every scanning of the printhead. However, 1-pass printing lowers the image quality because irregular density typically appears in an image owing to variations in discharge amount and discharge direction caused by the error of the orifice manufacturing precision, the error of the mechanical precision of the printing apparatus, and the like.
As a measure against irregular density, multi-pass printing is generally adopted. In multi-pass printing, pixels printed by one scanning are decimated. The printhead scans a plurality of number of times the same area (band) on a print medium that is narrower than an area covered by one scanning of the printhead, thereby forming an image. Multi-pass printing can reduce irregular density caused by variations in discharge amount and discharge direction because an image is printed in the same area by discharging ink from orifices which are different between scanning operations (passes).
However, if orifice arrays (head chips) corresponding to ink colors (types) are aligned bilaterally asymmetrically in the printhead scanning direction when the printhead reciprocally scans to print, the order of inks landed on a print medium changes depending on the printhead scanning direction. For example, in printing with dye inks, an ink landed first stays in the print medium surface layer. The color of an ink landed first influences the tint of an image greater than the color of an ink landed later. For this reason, in multi-pass printing, an ink color printed first by the first pass greatly influences the tint of the band. However, when orifice arrays aligned bilaterally asymmetrically and the printhead reciprocally scans to print, the ink color printed first by the first pass differs between bands. Areas with different tints alternately appear on a print medium, and the irregular density is recognized in the image. As a method of avoiding irregular density, Japanese Patent Laid-Open No. 5-27832 discloses a method of changing, for each print color, a mask pattern (pass mask) for dividing print data in correspondence with a plurality of scanning operations in multi-pass printing. The method disclosed in this reference can reduce the influence of irregular density because pixels with different ink landing orders can be distributed in any band on a print medium.
For a printer such as a large-format printer in which the printhead scans in a large width, it is necessary to increase the moving speed of the printhead for high-speed printing, and thus increase the ink discharge frequency. However, this method delays transfer of data (discharge pattern) which determines orifices for discharging ink.
One method of solving this problem is block decimation. According to this method, nozzles for discharging ink are divided into blocks, and the nozzle driving timing is shifted for each block. In this time division driving, blocks driven in one cycle (one column) are limited, and the printhead scans the same area on a print medium a plurality of times. As a result, the discharge pattern can be transferred in time. However, when performing block decimation, blocks (nozzles) permitted to be driven by the same pass are the same in all orifice arrays in the conventional arrangement in which orifice arrays are aligned bilaterally asymmetrically and the printhead reciprocally scans to print. Thus, no irregular density can be suppressed.